1. Field of the Invention
The invention relates to positron emission tomography (PET) scanners used to image areas of interest in patients, and particularly apparatus and methods for cooling PET scanner detector scintillation crystals, so that they are maintained at a stabilized temperature selected for desired detector performance.
2. Description of the Prior Art
PET scanners image areas of interest in patients who have ingested radioactive imaging solutions. The scanner utilizes an array of scintillation crystals in a generally annular scanner gantry to detect radioactive particle emissions from the patient; and then correlates those emissions with patient tissue structure. One known scintillation crystal material is lutetinium oxy-orthosilicate (LSO). In PET scanner operation, a radioactive particle emitted from the patient striking a detector crystal within a detector element causes a light emission. That light emission is in turn detected by a photomultiplier tube (PMT), charge coupled device or the like. The PMT in cooperation with a detector electronics assembly (DEA) converts the scintillation crystal's detected light emission to an electrical signal that is used by scanner to generate an image of patient tissue in the area of interest.
Light output of scanner scintillation crystals can be temperature dependent. As shown in FIG. 1, a lutetinium oxy-orthosilicate (LSO) crystal reduces effective light output (hence detector sensitivity) unless stabilized detector crystal temperature is maintained below 300° Kelvin (81° F. or 27° C.). Known PET scanners passively maintain detector crystal temperature. Components within the PET scanner gantry, for example the DEAs, generate and emit heat during their operation that is trapped within enclosed, sealed gantries. Ambient air ventilation is not commonly utilized in PET scanner gantries. Known PET scanners utilize water-cooled heat exchanger cooling rings in the sealed gantry structure to absorb and transfer heat away from the scanner. Cooling rings incorporated in the gantry structure rely primarily on convective heat transfer, assisted somewhat by forced exhaust fans incorporated within the DEA structures. Detector crystals are indirectly cooled by attempting to maintain the overall operating temperature within the gantry sufficiently low to meet the crystal operational temperature needs. However, localized temperature zones within the scanner gantry are not controlled with sufficient precision to assure constant operational temperature needed to maximize scintillation crystal detector sensitivity. Thus, the cooling ring heat transfer system is operated at a higher heat transfer output level than necessary to cool all gantry components generally, without assurance that the indirectly cooled detector crystals are being maintained at their optimum stabilized temperature threshold.